1. Field of the Invention
The present invention relates to an optical system and an optical device including the same, and is suitably applied to an optical device such as, for example, a silver halide camera, a digital still camera, or a digital video camera.
2. Description of the Related Art
An optical system for a use in an image taking apparatus such as a digital camera or a video camera is required to have a short total lens length (length from a first lens surface closest to an object side to an image surface), and to be small and lightweight as an entire optical system. In particular, an optical system with a large diameter is likely to have a long total lens length and a large weight, and hence it is strongly required to reduce the size and the weight. In general, when an optical system is reduced in size and weight, various aberrations, in particular, chromatic aberrations such as a longitudinal chromatic aberration or a lateral chromatic aberration is increased, with the result that optical performance is likely to be deteriorated. In particular, in a telephoto type optical system (telephoto lens) which is reduced in total lens length or weight, chromatic aberration is increased as the focal length becomes long.
As a method of reducing chromatic aberration in an optical system, there is generally well known a method in which an anomalous partial dispersion material is used as an optical material (see U.S. Pat. No. 6,115,188).
In the telephoto type optical system, chromatic aberration is corrected in a front lens unit at which passing positions of a paraxial on-axis light ray and a paraxial chief ray are relatively higher than that of the optical axis. Specifically, a positive refractive power lens made of a low dispersion optical material such as fluorite having anomalous partial dispersion (optical member having a large Abbe number) and a negative refractive power lens made of an optical material having high dispersion are used for correcting chromatic aberration. Here, the paraxial on-axis light ray refers to a paraxial light ray that enters in parallel to the optical axis of the optical system when a focal length of the total optical system is normalized to be one and the light ray has a height of one from the optical axis. Meanwhile, the paraxial chief ray refers to a paraxial light ray that passes through an intersection of an entrance pupil of the optical system and the optical axis, among light rays entering at −45 degrees with respect to the optical axis, when a focal length of the total optical system is normalized to be one.
For example, in U.S. Pat. No. 6,115,188 discloses a telephoto type optical system in which a low dispersion optical material such as fluorite having anomalous partial dispersion is used, to thereby reduce chromatic aberration. In the telephoto type optical system using fluorite as an optical material, it is easy to correct chromatic aberration in the case where the total lens length is set relatively large. However, when trying to reduce the total lens length, chromatic aberration increases.
The reason is as follows. That is, chromatic aberration generated in the front lens system is reduced by merely utilizing low dispersion and anomalous partial dispersion of the material such as fluorite. In order to correct chromatic aberration of the optical system that is deteriorated due to reduction of the total lens length, it is necessary to largely change a refractive power of a lens, for example, in a lens system using a low dispersion glass such as fluorite having a large Abbe number. Therefore, it is difficult to correct appropriately all aberrations including spherical aberration, coma aberration, and astigmatism, which are generated due to increases of chromatic aberration and a refractive power. In the optical system disclosed in U.S. Pat. No. 6,115,188, a low dispersion material having a high anomalous partial dispersion property is used at a position having a large front lens diameter so that various aberrations such as chromatic aberration can be appropriately corrected.